Rubber grease, always worked good for me in preventing BMW rubber bits from drying out and cracking. Used to be available at autoparts but now seems to be replaced everywhere with sillycone type greases and to me they don't seem to penetrate the rubber as well, stays greasy and attract dirt.
Looked around last year, my large tube was empty after 20 or so years. Found some on fleabay UK, cheap and I am good for another 20 years. But already gave some away in little jars, stuff goes a long way.